The present invention pertains to the vacuum tube arts, and in particular to a getter for maintaining vacuums. It finds particular application in conjunction with rotating anode x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in the generation of radiation and vacuum tubes for other applications.
Typically, rotating anode x-ray tubes include a sealed and evacuated envelope in which the cathode, anode, anode bearings, anode rotor, and other associated structures are sealed. To maintain the vacuum in x-ray tubes, getters are traditionally mounted inside the vacuum envelope to absorb stray gases. During manufacture, the tube components are cleaned and baked in vacuum furnaces. This procedure reduces the amount of surface contaminants available to evolve gases when the tubes are in service. The cleaned x-ray tube components are then assembled and placed within the envelope. The envelope is evacuated using a vacuum pump while it undergoes various heating processes that liberate absorbed gases from the surfaces of the internal parts. The envelope is then sealed to maintain the vacuum integrity.
The getter material is generally activated after the vacuum pump has exhausted the tube and the tube is permanently sealed off. Depending on the type of material, getters are classified as either evaporable or non-evaporable, e.g., a bulk getter. Activating the getter material may include flashing the getter for an evaporable material or actuating the getter by raising its temperature for a bulk or non-evaporable getter.
One method of activating (flashing) an evaporable getter is accomplished by locating a source of an electromagnetic field, usually an RF field, outside the evacuated envelope, proximate the getter wire/coil located inside the envelope. An electromagnetic field is generated and couples with the getter wire/coil, thereby inducing current flow in the wire/coil and heating the getter material in contact with the coil. The heated getter material evaporates and atoms, e.g. barium atoms, leave the getter wire/coil and are deposited on surrounding cooler interior surfaces of the envelope and other internal components. The freshly deposited getter material binds gases on its surface and/or absorbs residual amounts of such gases to maintain the vacuum state in the tube after it has been exhausted. This process of removing residual gases from an evacuated area by binding and/or absorbing is known as pumping.
Another method of activating the getter by flashing includes applying an electric current directly to the getter material via dedicated terminals. The getter is heated by resistance heating, thereby raising the temperature of the getter material to that necessary to evaporate the getter. This method is often used for in-service activation of gettering material, since the RF method is difficult to apply once the envelope has been installed in an oil-filled housing.
U.S. Pat. No. 5,509,045 to Kautz and U.S. Pat. No. 6,192,106 to Miller, et al., for example, disclose gettering systems for x-ray tubes.
In activating the getter material, the barium or other vapor condenses on adjacent cool surfaces. If the getter material vapor plates out on the glass envelope or x-ray tube electronic components, problems can ensue. The glass envelope is an insulator and the barium molecules are conductors which hold a static charge. These static charges can arc to other metal structures in the x-ray tube or to each other. This arcing can crack the glass, form short circuits to ground, or the like. Gettering material deposited on anything except grounded components can damage the x-ray tube. The gettering material is typically used in the form of a C-shaped wire with a channel with getter material packed in the channel portion of the wire, or in a ceramic package. This wire is firmly mounted within a cup which surrounds the cathode assembly. The cathode assembly surfaces are grounded via electrical feedthroughs, eliminating electrical arcing from the deposited getter material. The gettering material is contained within a cup getter shield and is positioned adjacent an integral part of the cathode assembly. The getter shield part of the cathode assembly holds the vaporized gettering material within the cathode assembly while still providing access for any stray molecules that need to be absorbed.
In evaporative gettering systems, the getter film produced by flashing reacts with all the residual active gases and, by chemisorption, removes them from the gas phase to reduce the gas pressure within the evacuated envelope. However, the sorption capacity of the deposited getter is limited. As sorption capacity is approached, the ability of the getter to absorb additional gas molecules is diminished. The components of the x-ray tube continue to evolve gases during service. As the getter material becomes saturated and less efficient in removing gas molecules, the pressure in the evacuated envelope increases. Over time, the gettering material reaches saturation, i.e., it can no longer absorb stray gases.
When the gas pressure within the evacuated region of the x-ray tube increases, the mean free path between gas molecules is reduced such that a chain reaction is more likely to occur when the gas molecules in the envelope are ionized by the high electric fields generated during normal tube operation. This chain reaction is termed an avalanche and is a form of arcing, which can lead to catastrophic failure of the tube. In x-ray tubes, this tendency to arc often increases as the tube ages.
One way to reduce this potentially damaging arcing is to refresh the gettering material during service. U.S. Pat. No. 6,192,106, for example, discloses a field-flashable gettering system in which an additional gettering coil is activated in the field when the conditions indicate that the main gettering system is reaching the end of its useful life.
For higher power x-ray tubes currently in use, the amount of gettering material surface area available for gas absorption is currently limited by the size of the cathode assembly getter shield cup. Additionally, the rate of pumping is dependent on the amount of available getter material surface. As tubes get larger, the limited amount of getter material available in the getter shield cup leads to long pumping times.
The present invention provides a new and improved x-ray tube using a getter shield and method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, an x ray tube is provided. The x-ray tube includes an envelope which encloses an evacuated chamber. The envelope includes an electrically conductive portion. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber and positioned to be struck by the electrons and generate x-rays. A gettering material is in electrical contact with the electrically conductive portion for absorbing stray gases within the chamber.
In accordance with another aspect of the present invention, an x ray tube is provided. The x-ray tube includes an envelope which defines an evacuated chamber. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber and positioned to be struck by the electrons and generate x-rays. A piece of gettering material is disposed adjacent an electrically conductive portion of the envelope, which, when activated, deposits a layer of gettering material on the electrically conductive portion.
In accordance with another aspect of the present invention, a method of removing gases from an evacuated chamber of an x-ray tube is provided. The method includes disposing a gettering material adjacent a conductive portion of an envelope which defines the chamber and flashing at least a portion of the gettering material such that a layer of the gettering material is deposited on the conductive portion of the envelope.
One advantage of at least one embodiment of the present invention is that an x-ray tube has an increased useful life.
Another advantage of at least one embodiment of the present invention resides in the ability to refresh an x-ray tube gettering system during service.
Another advantage of at least one embodiment of the present invention is that it enables x-ray tubes of larger evacuated volumes to maintain proportional increases in the gettering system.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.